Hi,
Outdoor concerts can be challenging to say the least. In order to do a multiband concert successfully, you should have stage plots of every band, as soon as humanly possible. I'm doing a concert this weekend, and I've had my plots for 2 weeks.
First determine how many change outs you need to do. Then schedule how much time to allow between acts for the change over. This can vary from 5 minutes to pull and place one acoustic guitarist to a half hour or more if you have large bands. The Dead air can be covered by a solo performer to stage left or right who plays through the breaks. This could also be the MC.
Where ever possible, pre-stage the equipment to the rear of the stagew. When you move it up, and while the musician is working with the sound man, you can pre-set the next act at the rear, to be moved up into place when the previous act isi finished.
Try to establish a flow, so that the equipment comes up on stage from one side, and goes down the other, That makes movement easier for everybody.
I f you are working with a rented sound system, that's the soundman's job. If not, and you are using a system from one of the bands, they should set it up, and run it. Don't let them leave the board when a conperformance is in progress.
If you are working with a rented backline, they will set everything up. If not, make sure you have a minimum of 4 quad AC drops. More is better.
Make up a schedule in advance and stick to it. If a band is screwing around with their equipment, make them aware of their start time, and if they take too long setting up, make sure they end on time. Don't penalize the following band for the actions of the preceding band.
Have lots of people available to haul the equipment. Discuss the placement of the equipment *before* you go onstage, then make sure the gear gets placed in the right positions.
Without a little more detail, I don't know what else to tell you except the following:
You're the boss, stay on schedule, don't let musicians or their "people" push you around.
If you could add some detail (how many bands, what kind of bands, rented or borrowed backline) I may be able to help you further.